fix(django 2.0): swallow exception kwarg in custom 404 view (#27090)

src/sentry/web/frontend/error_404.py

@@ -4,5 +4,7 @@ from sentry.web.helpers import render_to_response
 
 
 class Error404View(View):
-    def dispatch(self, request):
+    def dispatch(self, request, exception=None):
+        # HACK: We don't have any use for exception, but in Django 2.0,
+        #       signatures for 4XX handler views were changed to include it.
         return render_to_response("sentry/404.html", status=404, request=request)